There are many requirements for the cover of a golf ball, for example, abrasion resistance of keeping the ball surface from becoming fluffy by the friction caused by hitting clubs or on grounding, shot feeling, high controllability of approach shot, flight characteristics or rebound characteristics, durability, and the like. Balata cover, ionomer cover, urethane cover, and the like have been known and used as the cover of a golf ball.
Thread-wound golf balls having a balata cover or rubber cover are known to be excellent in shot feeling and controllability, but inferior in durability and more expensive. Accordingly, ionomer cover and urethane cover have been proposed as a cover for replacing the balata cover. It is often pointed out that golf balls having an ionomer cover are superior in durability but rather poor in shot feeling and controllability. Alternatively, golf balls having a urethane cover are better in shot feeling and controllability than golf balls having an ionomer cover, but slightly lower in durability. As described above, although many covers were proposed for improving the properties of golf-ball covers, completely satisfactory golf balls are currently not available.
In particular, numerous golf balls having ionomer cover or urethane cover have been proposed recently. However, a smaller number of improvements have been proposed with respect to golf balls having a rubber cover, for example, by U.S. Patent Application Publication No. 2003/0073512A1.
Injection molding of golf balls where a rubber composition is injection-molded around a core, for example, has been known as the method for manufacturing golf balls having a rubber cover. However, the golf balls produced by an injection molding method tend to have lower abrasion resistance due to the orientation of the rubber flowing in the mold during injection molding.
On the other hand, the method for producing golf balls by forming half-shell covers using a rubber composition and press-molding a core covered by two half shells has a disadvantage that unless the rubber composition is previously crosslinked to some degree, it is difficult to keep the shape of half shells and the thickness of the resulting cover layer uniform, leading to decrease in durability of the covers. If the rubber composition is crosslinked excessively for preserving the shape of half shells, the adhesion strength between the bonding surfaces of two half shells (adhesion surfaces) declines, leading to defects in half-shell bonding. Consequently, the durability of the resulting cover also declines.